This invention relates to abrasive diamond particles, and method for producing the same, suitable for the use as slurry suspended in a medium. Such particles are in particular useful for the texturing process of fixed disk blanks of nickel-coated aluminum.
The precision has remarkably improved over these years in the polishing and buffing process. Electronic industries, for example, are experiencing a rapid increase in the memory capacity of fixed disks, due mainly to decreased gaps between the disk and the magnetoresistive head, which is achieved as a result of improved surface precision of the recording mediums. Very minute diamond particles of especially sub-micron sizes are commonly employed for the polishing and buffing processes for such mediums and heads.
Micron-size and smaller diamond abrasive is used either as fixed in formed disks or as slurry of loose particles that are dispersed and suspended in a medium. Uses of slurry remain still common, although needs are rising for fixed particles for a better grit economy.
For slurry applications diamond of 5 xcexcm or less are commonly used and especially preferred are sub-micron particles which have an average size of 1 xcexcm or less.
In machining processes using a slurry, it is required that diamond particles remove the work material efficiently in relation with time. Also it is essential that the finished surface roughness be minimized, while least of them be left sticking on the surface of rather soft work material. In general, however, the first requirement cannot be achieved at the same time with the second or third.
In those circumstances polycrystalline type diamond particles are commonly favored for precision machining applications, in which primary particle crystallites, of tens of nanometers in size, cohere together to make up secondary particles of several micrometers.
Such polycrystalline diamond however is produced in general by dynamic compression techniques which are based on the detonation of much explosive, so many restrictions that are often imposed on the processes make the products too expensive for common applications.
Therefore one of the principal objects of the present invention is to provide a diamond abrasive of minute single crystal type, whereby, while diamond crystals from normal static compression processes is used as the starting material, those three requirements can be met at the same time and, further, controlled separately for adapting particular work materials.
In the invention diamond particles having a size of a few micrometers are heated and held at a temperature of 1000xc2x0 C. or more, either in a vacuum or in an atmosphere of inert gas such as nitrogen, argon and helium. Diamond is thereby converted in part, especially on the surface, to non-diamond carbon, which essentially comprises graphite, amorphous carbon and turbostratic carbon.
Minute cracks are formed in the heat treatment of the invention. This occurs probably as a result of the conversion to non-diamond carbon, which becomes promoted by the metallic occlusion and inclusion in molecular or atomic state within the diamond particle.
As employed for polishing applications the diamond particles of the invention can achieve a smoother finished surface, due probably to the combined effect of the amorphous carbon or graphite as lubricant and buffer for the intermittent loading when the abrasive particles come in contact with the work surface during a machining process.
Due to those internal cracks the particles of the invention, when under an excessive loading, are crushed, and chipped off, just within rather a limited zone in the vicinity of the cutting edges that have engaged with the work. This prevents the occurrence of significant scratches on the work surface as in the case of polycrystalline type, where primary or smaller particles gather firmly to make up a secondary or larger particle. Also effective are fragments that are off from the particle bulk, to yield a decreased roughness on the finished work surface.
Moreover fresh cutting edges take over automatically at the sites of chipping, in order to achieve an uninterrupted polishing process. That is an improved machining efficiency is attainable in terms of stock removal over a unit time per abrasive particle.
In the invention the conversion of diamond to non-diamond carbon (NDC) may be readily evaluated by wet oxidization technique, whereby a mixture of diamond with such diamond-turned carbon is intensely heated in concentrated sulfuric acid or concentrated nitric acid or their mixture. After the latter has been completely oxidized and removed from the outer surfaces and open cracks, the weight change thus caused over from the acid treatment is calculated to determine the carbon concentration or yield.
Raman spectroscopy is also available for evaluating the relative concentration of non-diamond carbon to diamond in the mixture. This technique permits a quicker determination at a higher sensitivity, although special equipment is required. Here we use for the index the ratio in peak height, or spectral intensity data acquired at a specific wave number that corresponds to graphite or amorphous carbon to diamond, the former occurring between 1500 and 1600 cmxe2x88x921 and the latter, in the vicinity of 1330 cmxe2x88x921.
In said spectroscopy the peak height is measured as such above the straight base line that passes the data points at 1200 and 1700 cmxe2x88x921 as the both limits of the observed wave length. Evidently the absolute data values inevitably may differ to a degree from equipment to equipment. Our data were obtained by using JASCO NR-1800 Raman Spectroscope.
Raman spectroscopy is an efficient technique for detecting non-diamond carbon, as its sensitivity is commonly 50 to 80 times higher than that for diamond. The benefits of this invention is attained, as observed by this technique, when diamond-turned non-diamond carbon is contained at a G-D index of 0.1 to 4.0, and more between 0.2 and 2.0, especially 0.3 and 1.0.
At an index of 0.1 or less such benefit is not remarkable that the stock removal is efficient and the surface roughness is insignificant. At 4.0 or more, on the other hand, the diamond particles have excessively decreased rigidity and become ineffective for the machining as not capable of achieving an adequate stock removal.
The diamond particles of the present invention, due to the specific heat treatment, exhibit a significant improvement in surface roughness of the finished work when employed for the machining of, at least, the materials described herein. As heat-treated the particles have a somewhat varied average size, and yield an 80% surface roughness relative to the same diamond lot as untreated, in the machining process for the specified work material. The surface condition is observed by atomic force microscopy (AFM).
For the heat treatment of the invention is available a temperature between 1100xc2x0 and 1400xc2x0 C. In particular good results can be obtained in both stock removal (machining efficiency) and surface roughness when treated at 1200xc2x0 to 1300xc2x0 C. Such temperature is maintained for 3 to 48 hours dependently upon the batch volume.
The treated diamond of the invention is in particular adapted for the texturing of fixed disks of nickel coated aluminum alloy as a recording medium for computers. In the machining with conventional diamond abrasive, without such treatment, a significant number (around four per disk in average) of particles are found left sticking to the work material, as a result of the contact too hard. None are observed on the work when processed with the diamond particles of the invention, in which an optimized proportion of non-diamond carbon is provided between the diamond and the work and also the diamond particles are imparted with proper friability.
The environment for the heat treatment is set non-oxidizing for diamond, which may be either a vacuum of pressure not exceeding 10 Pa or an inert gas atmosphere of argon, helium or nitrogen. For the purpose of assured treatment effect and economy, argon or nitrogen in particular can be filled in a hermetic chamber to a pressure a little positive over the outside atmosphere.
The heat treatment of the invention decomposes and removes for the most part various adsorbed chemicals on the particle surfaces. This is advantageous in that diamond particles have collected on the surface, during the production steps, chemicals such as sulfate and nitrate, which come from the dissolution process for the removal of the metallic contaminants from the crushing medium.
While such chemicals can be removed simply by washing with water if taking much time and work. Heat treatment is also available as an efficient technique for the complete removal. It is essential that in each case the combined concentration of the remaining radicals should be less than 5 p.p.m. or the detection limit for the ion chromatography.
The diamond particles as heat-treated by the method of the invention exhibit wettability to aqueous mediums significantly decreased, due to the deposition of non-diamond carbon on the surface yielded thereby. It is effective for imparting hydrophilicity to treat such carbon-coated diamond from the heat treatment in a wet oxidizer, such as mixture of sulfuric and nitric acid at a temperature of 100xc2x0 to 150xc2x0 C. and, preferably, to 120xc2x0 C. Hydrophilicity is attained thereby along with the removal in part of the non-diamond carbon on the surface.
For the wet oxidization described above and the evaluation of non-diamond carbon yield, these acids or oxidizers are available: H2SO4, HNO3 and HClO4, which may be used as the main component either singly or in combination. So the diamond typically may be treated in a mixture of sulfuric and nitric acid, heated at 120xc2x0 C. Thereby the non-diamond carbon in part is removed from the surfaces of diamond particles, and there formed are hydrophilic atoms and groups such as oxygen, hydroxyl, carboxyl and carbonyl in order to improve the wettability to aqueous mediums.
The wet oxidizer described above may further comprise another selected from KNO3, CrO3 and KMnO4.
Such atoms or groups can be provided on the particle surface by first halogenizing and then hydrolyzing. For example, diamond powder is placed in a reaction chamber and heated at 300xc2x0 C., to which chlorine gas is passed in order to chlorinate the surface of diamond and non-diamond carbon, then by putting said powder into water, in order to provide hydrophilic atoms or groups.
Besides the oxidization and chlorination/hydrolysis techniques, these are also available:
For the dispersion in an aqueous medium, breaking down aggregated diamond into individual particles by means of intense vibration or shock loads. For this purpose available are ultrasonic homogenizer, dispersion medium assisted automatic shakers, and ball mills of conventional type.
Use of certain surfactant to mediate between the diamond/carbon surface and water. For this purpose is selected one or more among a wide range of anionic, non-ionic, and polycarbonic products.
Combination of the above two. For example the stability of disintegrated particles can be increased by the addition of a surfactant to the water during the process with a homogenizer.
For the purpose of this invention, which contemplates the application to the precision machining, particle sizes of 5 xcexcm or less are suited as a starting diamond and sizes between 4 and 0.1 xcexcm are preferred. Coarser particles are not significant for the applications in slurry state.